Recently, Radio Frequency Identification (RFID) systems are widely used. Typically, some RFID systems utilize electromagnetic waves in a UHF band (900 MHz band) or a microwave (2.45 GHz) as a communication medium, and some RFID systems utilize a magnetic field generated by mutual induction. Among them, the RFID system utilizing electromagnetic waves in the UHF band is attracting attention since the RFID system can provide relatively long communication distance.
A micro-strip antenna is proposed as an antenna of a reader-writer which communicates with an RFID tag utilizing electromagnetic waves in the UHF band. The micro-strip antenna uses a micro-strip line as an antenna (see, for example, patent document 1 and non-patent documents 1 and 2).
There is a system which includes an antenna provided on a surface of a shelf. Merchandise to which an RFID tag is attached is arranged on the shelf. The system identifies that the merchandise is taken away from the shelf when the system becomes unable to detect the RFID tag. In such a system, it is preferable to use an antenna apparatus which can read the RFID tag attached to the merchandise provided in an area close to the surface of the antenna and can read the RFID tag over the entire surface of the shelf.
However, a communication distance of the conventional antenna is not sufficient and it is difficult to generate a uniform electric field over the entire surface of the antenna, particularly when size of the antenna becomes larger. Accordingly, it is difficult for the conventional antenna to provide uniform and sufficient communication distance.
Therefore, it is difficult to read all of the RFID tags uniformly in a case where a plurality of merchandise to which the RFID tags are attached is arranged on the shelf, in a case where the conventional antenna is used in the system as described above.